Synthesis And Properties Of Bipolar Host Materials Based On Cyanopyridine And Triazole Blocks

Owing to the merits of low power consumption, high contrast, self-luminescence, super thin and flexibility, OLEDs have gradually become the top contender as the new generation flat-panel technology and solid-state lighting source. As the core part of the OLED, the structure design of light-emitting layer is critical. Currently, almost all luminous layers adopt doping structure with host and guest. Due to more balanced carrier transportation than the unipolar counterparts, bipolar host materials become the optimal choice for host materials. It makes the excitons recombine in the whole light-emitting layer, then highly improve the efficiency when the bipolar host material is used.Carbazole and its derivatives are a class of excellent hole transport materials with high triplet energy. They are widely used in bulding up P-type charge transporting materials and light-emitting ones. Triazole, cyano and pyridine are strong electron-withdrawing groups, and are widely used as n-type units to construct electron transport materials. In this thesis, carbazole was selected as p-type units, and triazole and cyanopyridine as n-type groups.to design bipolar host materials for application in OLEDs. Seven new bipolar host compounds, that is m-CzPyCN, o-CzPyCN,n-CzPyCN, t-CzPyCN, DzSCz, DzDCz and SzDCz, have been synthesized and studied.To gain a high triplet energy, the effective molecular conjugation was controlled via the ortho and meta substitution strategy with large steric hindrance, as a result the donor and acceptor in the molecule were spatial isolated. Their HOMO and LUMO energies of the resultant compounds were determined by cyclic voltammetry and absorption edge technique. Density functional theory calculation was carried out to optimize the conformation of these molecules and to show the orbitals distribution. It is observed that HOMO and LUMO are almost completely separated for each compound, with HOMO mainly located in electron donating units and LUMO on electron withdrawing groups respectively. The triplet levels of these compounds are high enough to act as hosts for green emitter and blue emitters.Organic light-emitting diodes were fabricate with Ir(ppy)3 and FIrpic as geen and blue doped emitter in these hosts, respectively. For green devices with Ir(ppy)3 doped in cyanopyridine type moleculars, o-CzPyCN has the most outstanding performance, with a maximum current efficiency reaches of 60 cd/A. For the blue devices with FIrpic doped in triazole type hosts, SzDCz host exhibits the best performance, with the maximum current efficiency, power efficiency and external quantum efficiency of 27.8 cd/A,21.9 lm/W and 12.9%, respectively The present results indicate that not only the substitution style of ortho-, meta-, meta-, and para- and thus the molecular conformation, but also a proper ratio of donor and acceptor in molecule, are important factors to determine the overall performance of bipolar host materials in OLEDs.